Preparation And Application Of Water-soluble NaYF 4 :Yb,Er Upconversion Nanoparticles

Upconversion (UC) luminescence materials is an important class of rare earth luminescent materials, which can convert near-infrared (NIR) light to visible light by emitting higher energy photons after absorbing lower energy photons. Upconversion is an anti-Stokes process where long wavelength radiation is converted to shorter wavelength radiation via a two-photon or multi-photon mechanism. Upconversion luminescence materials have great applications in compact solid-state lasers, infrared quantum counter detectors, temperature sensors, solar cells, three-dimensional presentation, anti-counterfeiting technology, and immunology.In recent years, lanthanide-doped upconversion nanoparticles (UCNPs) have been developed as a new class of luminescent optical labels that have become promising alternatives to organic fluorophores and quantum dots for applications in biological assays and medical imaging. Compared to conventional organic dyes and quantum dots, UCNPs possess many advantages, such as low excitation energy, narrow emission peak, large Stokes shifts, high chemical stability, tunable maximum emission wavelength with changeable compositions and low toxicity. In addition, The UCNPs utilizes near infrared (NIR) excitation rather than ultraviolet (UV) excitation, thereby significantly minimizing background autofluorescence, photobleaching, and photodamage to biological specimens. It also allows in vivo observation with substantially high spatial resolution and offers remarkable sample penetration depths that are much higher than those obtained by UV excitation. Therefore, UCNPs have great application prospects as luminescent optical labels in biological assays and medical imaging.Among the so many commonly used UC host materials, including oxides, oxysulfides, fluorides, phosphates and others, the hexagonalβ-phase NaYF₄ is one of the most efficient host materials, and most of the attention has been focused onβ-NaYF₄ UCNPs doped with Yb-Er or Yb-Tm rare-earth (RE) ion couples. Recently, β-NaYF₄:Yb,Er UCNPs were directly synthesized in organic media, which had excellent morphology and optical properties. However, the biological applications of UCNPs are greatly restricted because of their poor solubility in aqueous solution. To realize their potentials in biological applications of UCNPs, people have focused on the study on transferring the UCNPs from organic phases to aqueous solutions.It is well known that Au nanoparticles (NPs) have absorbance in UV region. 15nm Au NPs have a characteristic absorption band centered at 520 nm with a high molar extinction coefficient (εat 520 nm). The fluorescence resonance energy transfer (FRET) process is highly efficient when there is an appreciable overlap between the emission spectrum ofβ-NaYF₄:Yb,Er UCNPs donor and the absorption spectrum of the Au NPs acceptor. FRET is widely used for the research in molecule structures, properties, reaction mechanism and quantitative analysis.In this study, reviews were given on the development of nano-materials and nano-probe, the synthetic methods of UCNPs. The novel optical properties and potential applications of UCNPs were discussed. On the bases of mentioned above, we carried out three aspects of investigation:1,Oleic acid stabilizedβ-NaYF₄:Yb,Er UCNPs were synthesized by solvo- thermal method, and successfully transferred these UCNPs from organic phases to water solutions through the formation of an inclusion complex between surface-bound surfactant molecules andβ-cyclodextrin (β-CD).2,CDen were synthesized, and obtained water-soluble NaYF₄:Yb, Er UCNPs with amino-group.3,D-fructose was determined by FRET between Au NPs and NaYF₄:Yb, Er UCNPs.4,NaYF₄:Yb,Er nanorods were synthesized by hydrothermal method, and explored the conditions of coating mesoporous silica.